Semiconductor crystal growth from a domical projection



R. M. PIERSON Dec. 13, 1966 SEMICONDUCTOR CRYSTAL GROWTH FROM A DOMICAL PROJECTION 2 Sheets-Sheet 2 Filed Dec. 23 1963 INVENTOR. Wa e/K /77. Wrson ATTORNEY United States Patent 3,291,574 SEMICONDUCTOR CRYSTAL GRGWTH FROM A DOMICAL PRGJECTHBN Russell M. Pierson, Kolromo, Ind, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Dec. 23, 1963, Ser. No. 332,733 6 Claims. (Cl. 23-301) This invention relates to crystal growth and more particularly to a method and apparatus for growing monocrystalline semiconductor bodies of accurately controlled cross-sectional dimensions.

The present invention permits one to rapidly grow a monocrystalline semiconductor rod of a very precisely controlled cross section. It permits one to grow a monocrystalline rod that need only be sliced into appropriate wafer thicknesses. Consequently, it is a much more inexpensive way of obtaining the individual semiconductor wafers used in signal translating devices such as germanium transistors and the like. In addition, the present invention provides monocrystalline semiconductor waters of higher quality than that previously obtained. For example, germanium wafers exhibit no grain boundaries and have a dislocation density less than about 5,000 per square centimeter, as opposed to about 15,000 to 20,000 centimeters in conventionally obtained germanium wafers.

It is, therefore, an object of the present invention to provide both a method and an apparatus for the production of improved semiconductor bodies of accurately controlled cross-sectional dimensions.

These and other objects, features and advantages of the invention will become more apparent from the following description of preferred examples thereof and from the drawings, in which like reference numerals represent similar parts, wherein:

FIGURE 1 shows a schematic sectional view of an apparatus for growing crystals in accordance with the invention;

FIGURE 2 shows an enlarged schematic fragmentary view of the solid-liquid inter-face between a melt and the monocrystalline body being grown;

FIGURE 3 shows a view analogous to FIGURE 2 but prior to contact between a seed crystal and the melt;

FIGURE 4 shows a view along the line 4-4 of FIG- URE 1;

FIGURE 5 shows an elevational view of another crucible COIVF such as shown in FIGURE 4;

FIGURE 6 shows a sectional view along the line 66 of FIGURE 5;

FIGURE 7 shows an alternate crucible which can be used in place of that shown in FIGURE 1; and

FIGURE 8 shows a sectional view along the line 3-8 of FIGURE 7.

The objects of my invention are attained by forcing a domical projection of a melt for growing monocrystalline bodies through an aperture, contacting the domical projection with a seed crystal and pulling a single crystal from the domical projection while simultaneously regulating the contact area between the single crystal and the domical projection. Hence, the single crystal is drawn from the upstanding domical projection of the melt rather than from the entirety of the melt. Moreover, the invention also encompasses the regulation of the contact area between the solid single crystal and the melt domical p-rojection to accurately control the cross-sectional dimension of the single crystal being grown. In a preferred embodiment of the invention, the domical projection is formed by placing a weighted melt cover having a central aperture in it on the surface of the melt to force a domical projection of the melt through the aperture in the cover.

3,291,574 Patented Dec, 13, 1956 The distance between the solid-liquid interface and the cover surface, melt projection zone length, is monitored and regulated to control the size of the crystal grown.

The invention can be more easily described by reference to the drawings and for this reason attention is drawn to FIGURES 1 and 4. FIGURE 1 shows a cylindrical clear fused quartz furnace tube 10 having end closures 12 and 14 iorming a furnace chamber End closures 12 and 14 have annular silicone rubber seals 16 and 18, respectively, for sealing the opposite end surfaces of tube 10. A graphite crucible 20 containing germanium melt is supported on a quartz tube 22 which rests on end closure 14. A graphite melt cover 24 having an aperture 26 rests on the surface of the germanium melt. Weights 2'8 on the cover member 24 induce a projection 34 of the germanium melt up through cover aperture 26. A single crystal depending from a seed crystal is in contact with the projection 30. The seed crystal is supported by a chuck 32 which is connected to an appropriate pulling mechanism (not shown) for growing monocrystals. The furnace end closure 12 has a central opening 34- therein to allow entrance of the seed crystal chuck 32 into the furnace chamber. The aperture 34 is effectively shielded by means of a protective gas curtain produced by diametrically opposed gas jets 3-6 and 38. A protective atmosphere is maintained within the furnace chamber by continuously introducing a protective gas, such as nitrogen, hydrogen, argon, helium or the like. Tube 40, located in the lower end of the furnace chamber, serves .as a protective gas inlet. The crucible 20 in the furnace chamber can be heated by a radio [frequency induction heating coil 42 located outside the furnace tube 10. Suitable temperature sensors (no-t shown) are appropriately located to precisely measure melt temperature. If it is desired to use resistance heating instead of induction heating, the crucible -20 need not be graphite but can be quartz, and if of graphite, the resistance heating unit should be located within, rather than outside the furnace tube It In such instance, the tube 10 can be stainless steel. A small quartz window 44 in the side of the tube 10 allows observation of the interior of the assembly to monitor the melt-crystal interfacial area during crystal growth. A telescope 46 secured within the recess of the window area facilitates observation. A graduated scale (not shown) within the telescope is calibrated tor accurate measurements of the distance between the melt-crystal interface and the upper s-untace of the cover for accurate control of the crystal diameter.

In order to grow a monocrystalline germanium rod of accurately controlled cross-sectional dimensions in accordance with the invention, the crucible 20 is appropriately charged with germanium prepared in the normal and accepted manner. For example, 40 ohm-centimeter germanium ingot is etched in an equi-molar mixture of nitric acid, acetic acid and hydrofluoric acid. It is then rinsed with deionized water, rinsed with methanol and dried. The germanium ingot is then placed in the crucible, along with suificient impurity to suitably dope the melt, it concurrent doping is desired. An impurity content on the order of approximately 1 part per million is frequently employed. The crucible cover 20 is then placed over the ingot and the weights 28 placed on the crucible cover 20. The germanium charge, of course, is prepared in such a way that the cover and weights retain their proper disposition a' ter the charge is melted. For example, a single ingot of germanium having a broad, flat upper surface can be used. On the other hand, the arrangement shown in United States patent application Serial Number 332,706, filed December 23, 1963, entitled Crystal Growth, filed simultaneously herewith in the name of Hubert G. Dohmen, can be used to support cover 20 away from the change until the charge is molten.

The furnace is then closed and purged. The usual furnace gases for crystal growing can be employed including inert gases, nitrogen-hydrogen mixtures, substantially pure hydrogen or the like. In some instances it may be desired to use a vacuum. The charge is then melted. It is initially held at a temperature somewhat above the usual growing temperature. If appropriately weighted, the crucible cover 24, with its weights 28 thereon, will be supported on the surface of the melt forcing a domical projection 30 of the melt up through the cover aperture 26.

For a one and one-half degree inward tapered crucible having a maximum inner diameter at its mouth of approximately 2.156 inches, a crucible cover having an outer diameter of approximately 2.152 inches and a 0.04 inch circular central aperture can be used. The lower surface of the cover member is inwardly tapered at about six degrees to an edge thickness of approximately 0.01 inch around the central aperture, forming a generally convex or frusto-conical surface on the underside of the cover. Weights comprising about 46 times the weight of germanium displaced, e.g., 50 grams, are generally preferred.

A seed crystal having a 1,1,1 plane parallel to the surface of the melt is brought into close proximity of the domical projection for preheating. The melt temperature is retained at 5 C.l0 C. above its melting point. After approximately five minutes, the seed is lowered into the domical projection for a slight melt-back. The temperature is then dropped in small increments until precipitation of the melt commences on the seed crystal. The pull mechanism'is then initiated to slowly withdraw the seed crystal from the melt as the crystal grows without breaking contact between the solid monocrystalline body and the melt projection. If desired, the speed of the pull mechanism is then increased slightly to reduce the contact area between the domical projection and the single crystal to initially reduce crystal diameter as much as is practical before growth at the desired diameter to improve crystal quality. The speed of pull is then gradually increased, along with appropriate temperature adjustments until the desired rate of pull has been obtained. Temperature is continually adjusted, gradually dropped, as the speed of pull is increased.

The projection should be in the center of the heat field or the crystal will not grow symmetrically. Hence, during these preliminary steps, the heat field is adjusted to center it on the projection 30. If not, an elliptical crystal may result. A similar result is attained if the direction in which the crystal is pulled is not substantially vertical.

When the appropriate melt-crystal interfacial diameter is attained at the desired speed of pull, the temperature and speed of pull are held constant for substantially the balance of crystal growth. Interfacial diameter is measured by measuring the zone length between the meltcrystal interface and the upper surface of cover 24. Appropriate adjustment of temperature is thereafter made to attain the desired zone length; hence, crystal diameter. When this condition has been substantially stabilized, both temperature and speed of pull are held constant for substantially the balance of crystal growth. To terminate crystal growth, the speed of pull can be increased to break contact between the crystal and the domical projection. Once this contact is broken, crystal withdrawal is terminated so as to avoid rapid quench of the freshly grown end portion of the crystal. The furnace is then shut down in the normal and established procedure for furnace shutdown in germanium crystal growth.

High quality crystals of about 0.358 inch, plus or minus 0.005 inch, can be consistently grown in this manner at rates up to 15 inches per hour. However, at rates in excess of 12 inches per hourquality begins to diminish. For the faster rates of growth, it is preferred to grow at a constant temperature and final regulation of diameter control effected by slight changes in pull speed, just the opposite from that described in the preceding example.

Thus, either, or both, diameter control techniques can be used to arrive at the constant speed-constant temperature condition that produces constant diameter in the crystal being grown.

As can be seen more clearly in connection with FI URES 2 and 3, in the growing of a single crystal in accordance with my method, the seed crystal is placed in contact with the domical projection which upstands from the upper surface of the cover 24. The diameter of the crystal which is being grown will vary merely by varying the area of contact between the crystal and the domical projection. I have found that changes in the zone, particularly its length, between the surface of the cover 24 and the melt-crystal interface, accurately reflects size changes in interfacial area. Consequently, I prefer to monitor zone length and regulate zone length during crystal growth to accurately control diameter of the crystal being grown. Since there is a significant difference in color and brightness between the graphite surface and the solid seed crystal, the annulus formed by the surface of the projection can be accurately located and measured. However, monitoring of the change in the angle of the Zone with respect to the cover surface, or the crystal pull axis, can also be used, as is described in United States patent application Serial Number 332,708, filed December 23, 1963 entitled Control of Crystal Size, filed simultaneously herewith in the names of Hubert G. Dohmen and Herbert A. Sims.

As can be seen in connection with FIGURES 5 and 6, the aperture in the crucible cover need not be circular if a monocrystalline rod of generally square cross section is desired. Best crystal conformity is attained by a small zone length with this cover. An aperture configuration along the lines of that shown in FIGURE 5 is best em ployed to obviate the effects of surface tension. As can be seen in FIGURE 6, the cover has a cross section substantially the same as that shown in connection with the cover shown in FIGURE 1. However, in addition, this cover also contains smaller surface indentations 48 for the inclusion of additional weights should they be desired.

Analogously, the crucible configuration can vary to some extent. As shown in FIGURE 1, the crucible may have a slight inward taper from top to bottom. Of course, in such a crucible, the cover 24 will have a sufficiently small diameter to allow the cover to drop into the crucible to a sufiicient depth. The space between the outer periphery of the cover and the inner periphery of the top of the crucible can be varied without objectionable results. Since germanium surface tension forces tend to stabilize the cover within the crucible, the melt projection maintains its position very well. On the other hand, if desired, the upper portion of the crucible can be untapered with the bottom half of it having an inward taper, as can be seen in connection with FIGURES 7 and 8 Of course, the inward taper on the crucible is desired for crucible protection during freeze-out of the melt when growth has been terminated.

The height of the domical projection 30 is increased in a plurality of ways. The lower surface of. the cover mem ber is of a generally concave o-r frusto-conical configuration. Hence, the cover edge adjacent the aperture is extremely thin. Moreover, if the lower corner of this thin edge is rounded, as shown in FIGURE 3, additional projection height can be increased so that the projection extends substantially from the upper surface of the cover, not the lower surface. However, of greatest importance in forming the projection is the impression of a force on the germanium to push a domical projection through the aperture to useful height. As described, this force can be applied by weights 28 placed on the upper surface of the cover, as shown in FIGURES l and 4. Since the surface tension forces of germanium are extremely strong, much more weight can be placed on the cover than one would ordinarily expect. I prefer to employ weights equal to about 4-6 times the weight of germanium displaced by the cover. In general, an empirical formula which can be used to approximate the weight to be employed for any given crucible-cover combination is determined by the following formulae: (I) P=4t/r,

where t=surface tension, and

r=radius of the cover aperture. (II) P"=Weight of cover assembly/ area of cover surface in contact with germanium.

P should be larger than P". As a safety factor, P" of Equation II is generally held to be approximately twothirds of P. For greater accuracy, the buoyancy effect of the germanium displaced can also be included in P. However, it is preferred to experimentally optimize the weight used to obtain the safest maximum projection height for any crucible-melt cover combination used.

It is understood, of course, that while the melt projection 30 is formed in the described embodiment of the invention by means of a Weighted melt cover, the projection can be forced through the aperture in a variety of ways. For example, the hydrostatic pressure of a molten head of germanium in a reservoir can be used to'induce a projection of germanium through an aperture in the end of a closed tube. Analogously, a germanium melt can be contained in a cylinder containing a piston that is moved to urge the melt through an aperture in the end wall of the cylinder. Similarly, other means can be used to apply a force to the upper surface of the cover on the surface of the melt.

Other modifications of the invention will be obvious to those skilled in the art and, for this reason, it is understood that the preceding description is intended only to serve as an illustration of the main features of the invention. There is no intention to be limited thereby except as defined in the appended claims.

I claim:

1. The method of growing a monocrystalline body of controlled cross-sectional dimensions, said method comprising the steps of forming a melt in an environment for growing single crystals from the melt, placing said melt in contact with a member having an aperture, said member being substantially inert to its environment, forming a domical projection of said melt extending up from said aperture beyond the upper surface of said member, said domical projection filling the aperture and having a maximum cross-sectional linear dimension no greater than the aperture, contacting said domical projection with a seed crystal to establish a substantially frusto-conical melt zone upstanding above the surface of said apertured member with said melt zone having a solid-liquid interface, pulling a single crystal from said melt zone by progressive unicrystalline solidification of said melt on said seed crystal at said interface, maintaining said frusto-conical melt zone of predetermined size and shape during said crystal growth, maintaining said interface a predetermined height above the aperture during crystal growth, and maintaining the cross-sectional dimension of the frusto-conical zone no greater than the aperture and filling the aperture during said crystal growth by adjusting any one or more crystal growth conditions such as the rate of pull, inclination of pull axis, location of pull axis in said frusto-conical melt zone, location of said frusto-co-nical melt zone in the melt heat field, and melt temperature.

2. The method as defined in claim 1 wherein said aperture is in a uniformly weighted disk floating on the surface of said melt and said domical projection is forced up through the aperture by the pressure of the weight added to the disk.

3. The method as defined in claim 1 wherein the melt is formed of a semi-conductor and the aperture in the melt contacting member is circular.

4. The method as defined in claim 1 which also includes the step of maintaining the solid-liquid crystal growing interface substantially horizontal during crystal growth.

5. The method as defined in claim 4 wherein the melt is of a semiconductor, the aperture in said apertured member is circular, and a uniform cylindrical monocrystal is grown, after initial adjustments, at a constant rate of pull.

6. The method as defined in claim 4 wherein the melt is of a semiconductor, the aperture in the apertured member is circular, and a uniform cylindrical monocrystal is grown, after initial adjustments, at a constant melt temperature.

References Cited by the Examiner UNITED STATES PATENTS 2,944,875 7/1960 Leverton 23-273 X 3,025,146 3/1962 Runyan 23-301 X 3,077,384 2/1963 Enk et al 23-273 X 3,078,151 2/1963 Kappelmeyer 23-273 X 3,093,456 6/1963 Runyan et al 23301 X OTHER REFERENCES Floating Crucible Technique for Growing Uniformly Doped Crystals, W. F. Leverton, Journal of Applied Physics, vol. 29, August 1958, pages 1240-1244.

NORMAN YUDKOFF, Primary Examiner.

G. P. HINES, Assistant Examiner. 

1. THE METHOD OF GROWING A MONOCRYSTALLINE BODY OF CONTROLLED CROSSS-SECTIONAL DIMENSIONS, SAID METHOD COMPRISING THE STEPS OF FORMING A MELT IN AN ENVIRONMENT FOR GROWING SINGLE CRYSTALS FROM THE MELT, PLACING SAID MELT IN CONTACT WITH A MEMBER HAVING AN APERTURE, SAID MEMBER BEING SUBSTANTIALLY INERT TO ITS ENVIRONMENT, FORMING A DOMICAL PROJECTION OF SAID MELT EXTENDING UP FROM SAID APERTURE BEYOND THE UPPER SURFACE OF SAID MEMBER, SAID DOMICAL PROJECTION FILLING THE APERTURE AND HAVING A MAXIMUM CROSS-SECTIONAL LINEAR DIMENSION NO GREATER THAN THE APERTURE, CONTACTING SAID DOMICAL PROJECTION WITH A SEED CRYSTAL TO ESTABLISH A SUBSTANTIALLY FRUSTO-CONICAL MELT ZONE UPSTANDING ABOVE THE SURFACE OF SAID APERTURED MEMBER WITH SAID MELT ZONE HAVING A SOLID-LIQUID INTERFACE, PULLING A SINGLE CRYSTAL FROM SAID MELT ZONE BY PROGRESSIVE UNICRYSTALLINE SOLIDIFICATION OF SAID MELT ON SAID SEED CRYSTAL AT SAID INTERFACE, MAINTAINING SAID FRUSTO-CONCIAL MELT ZONE OF PREDETERMINED SIZE AND SHAPE DURING SAID CRYSTAL GROWTH, MAINTAINING SAID INTERFACE A PREDETERMINED HEIGHT ABOVE THE APERTURE DURING CRYSTAL GROWTH, AND MAINTAINING THE CROSS-SECTIONAL DIMENSION OF THE FRUSTO-CONICAL ZONE NO GREATER THAN THE APERTURE AND FILLING THE APERTURE DURING SAID CRYSTAL GROWTH BY ADJUSTING ANY ONE OR MORE CRYSTAL GROWTH CONDITIONS SUCH AS THE RATE OF PULL, INCLINATION OF PULL AXIS, LOCATION OF PULL AXIS IN SAID FRUSTO-CONICAL MELT ZONE, LOCATION OF SAID FRUSTO-CONICAL MELT ZONE IN THE MELT HEAT FIELD, AND MELT TEMPERATURE. 